Hard drive access time is one of the most limiting aspects of modern computing systems. Access time includes seek time, rotational latency, and command processing overhead time. Track-to-track seek time may be defined as the amount of time it takes a hard drive's read/write head to locate the disk track of a piece of data. Rotational latency may be defined as the average time for the sector being accessed to rotate into position under a head, after a completed seek. Rotational latency may be calculated from the spindle speed, being the time for half a rotation. A drive's average access time may be defined as the interval between the time a request for data is made by the system and the time the data is available from the drive.
One way to decrease data access time is to increase the rotational rate of the disk and hence decrease rotational latency. However, there are limits to how fast a drive can be made to spin without degrading performance, and thus data access time can be made only so fast by increasing spin rate. Today's hard drives are capable of amazing spin rates. For example, manufacturers have introduced drives that spin at over 10,000 rpms. However, at this spin rate tremendous heat is generated. As excessive heat will clearly degrade performance, the heat must be removed, which can complicate the design and add cost. Thus, it is desirable to improve access time without resorting to increasing the rate of disk rotation.
Another technique for improving access time is by integrating multiple devices together as in a RAID (redundant array of individual drives). In this technique a byte of data may be written and read as one bit to eight different drives. However, this substantially increases the cost of the system and is generally only appropriate for systems such as high-end servers.
A technique for increasing system capacity is data compression. However, this results in loss of performance because data become fragmented. For example, when data that are stored compressed are re-written they may not compress sufficiently to fit back into the sectors in which they were originally stored. In this case, the data become fragmented because they are stored in some random location. Hence, system performance is negatively impacted.
Therefore, it would be advantageous to provide a way to improve hard drive access time. It would also be advantageous to provide a way that does not increase the heat generated by the hard drive or require added cost or added complexity to the design to remove excess heat generated by the rotation of the disk.